This invention relates generally to large-scale liquid storage tanks, and more particularly to full-contact, floating-roof storage tanks used for storing liquids at atmospheric pressures. Such tanks, which sometimes include a separate, fixed roof, commonly range from 15′ to 400′ or more in diameter, holding up to 1.5 million barrels of liquid or more.
Vapor control is often an issue. Vapors escaping from stored hydrocarbon-based liquids can present a health, safety, or fire hazard. Vapors escaping from flammable liquids can form an explosive mixture with air. Other liquids, particularly those containing sulfur, have an objectionable odor when allowed to freely evaporate. Consequently, efforts are often made to minimize evaporation losses in storage tanks.
A floating roof is a buoyant structure that floats on the liquid surface, limiting evaporation. An “internal” floating roof is used inside a tank with a separate, fixed roof. An “external” floating roof is used in a tank that has no fixed roof. In addition to reducing evaporation losses, floating roofs also keep weather and airborne contaminants out of the stored product.
There are different types of floating roofs. A vapor-space roof typically has buoyant members that support a deck above the liquid surface. For example, some floating roofs have a relatively thin aluminum deck that is supported by members that float on the surface of the stored product, leaving several inches of vapor space between the surface of the liquid and the deck. The space is useful because aluminum decks are more subject to leaking than welded steel decks. The distance from the top of the roof to the bottom of the buoyant members can be relatively large, on the order of 12 inches or more. One problem with this arrangement is that the stored product often leaks into the floating members, and is difficult to remove without supporting the floating roof from a fixed roof that has been designed for this additional load.
Full-contact floating roofs, on the other hand, leave no space between the deck and the surface of the product. They are designed to float on the surface of the product.
In a typical floating-roof tank, the shell of the tank is cylindrical and the roof floats upon the surface of the liquid product stored in the tank, rising or falling within the tank as liquid product is pumped in or drawn out. To allow space for inlet or outlet piping or internal process structures, and to make it easier to perform maintenance work on either the floor of the tank or the bottom of the floating roof, structure is generally provided to keep the roof suspended off the floor when the tank is completely emptied.
Sometimes this suspending structure takes the form of supports that maintain the roof at a low level. These supports can be fixed or adjustable. Fixed supports provide limited maintenance accessibility and decrease the useable tank volume. Manually-adjustable supports impose less limitation, but generally require personnel to enter a confined, potentially dangerous space to adjust the settings of the supports. They also add more weight to the floating roof and create potential emission pathways.
Supporting a floating roof from a fixed roof or providing remotely-activatable bottom supports would allow the position of a floating roof to be adjustable from the outside. However, the cost of such arrangements has generally been prohibitive on full-contact roofs.
The fact that liquid product sometimes collects on the deck of the floating roof contributes to the high cost. If too much liquid reaches the top of the deck, it can imbalance the roof and cause it to sink. Recovery of a sunken roof can be expensive and time-consuming, and is a safety hazard. The added weight of liquid on the deck must also be factored into design considerations. The roof itself and the supporting structure must be strong enough to withstand the load of trapped liquid when the roof is in a suspended position. (U.S. standards and regulations today require designers to assume a live load of 12.5 psf on the floating roof.) These requirements have generally led to heavy roofs which, in turn, have led those skilled in the art away from idea of trying to suspend a full-contact floating roof from either a fixed roof or from the shell of the tank.
With vapor-space roofs, the problem of liquid on the top of the deck is sometimes solved by providing drain openings in the deck. However, conventional drain openings are not practical on full-contact floating roofs because the top surface of the deck is generally below the surface level of the stored product, and the stored product would tend to flow up through the drains onto the deck.